(1) Field of the Invention
This invention relates to an improved semiwide angle objective lens for use with single lens reflex cameras.
(2) Description of the Prior Art
In recent years, there has been an increasing trend towards compact size single lens reflex cameras and, along with this trend a requirement has developed for more compact and wider angle interchangeable lens. The shortest focal length of a standard lens for a 135 mm film format single lens reflex camera has been commonly chosen at 50 mm. However, lens shutter cameras have lenses with approximately equal to 40 mm are very popular. Thus, for SLR cameras, a semi-wide angle objective lens having a value of about f=40 mm has become a standard lens.
A desirable semi-wide angle objective lens having a compact size would have a preferred value of f=40 mm, an F number of F 2 to F 1.8 and a simple lens configuration of six lens elements or so. It has been extremely difficult to apply prior art lenses (e.g., Gaussian type six element lenses) for this purpose and keep the lens back focal length large enough for a single lens reflex camera and also provide a field angle up to about 56.degree..
The reason why it was so difficult is that keeping the back focal length f.sub.B long requires a smaller setting of the positive power of the front group, resulting in an increased positive power of the rear group making it very difficult to correct residual aberrations with the positive lenses of the rear group.
Another reason for the above mentioned difficulty is that the requirement for a large aperture ranging from F 2 to F 1.8 results in a decreased power of refraction at each lens surface, which leads to difficulty correcting the curvature of the sagittal image surface, making it impossible to maintain a wide angle of view.
Further, even if an attempt is made to obtain a semi-wide angle objective lens by utilizing a lens system of a so-called retrofocus type e.g. a Gauss type or modified Tessar type in which a negative lens is added to the front lens element, the total length of the lens system is increased, or the front lens element is increased in size and the lens is no longer compact.
Because of the reasons mentioned above, it has been extremely difficult to provide a six lens element semi-wide angle objective lens of simple configuration having a large aperture and a compact size.